1. Field of the Invention
This invention relates generally to platform wheelchair lifts and, more particularly, to a method for detecting weight on a platform wheelchair lift prior to stowage.
2. Description of Related Art
Wheelchair lifts typically include mobile platforms to raise and lower passengers between a loading position at a ground level and an entry position at the vehicle's floor level. Further, the wheelchair lifts are usually collapsible for storage, i.e., stowed, within the vehicle. Thus, the wheelchair lifts may include a load platform that is driven through motion patterns to attain loading, entry, and storage positions. In one type of wheelchair lift, the lift is provided with a parallelogram structure to carry a platform that receives the wheelchair. The lift also includes a hydraulic system for actuating the platform through the parallelogram mechanism and a control unit for controlling the different motion patterns to raise and lower the platform and to collapse the unit for storage.
Due to safety considerations, the wheelchair lift unit must be able to detect weight on the platform before collapsing the unit for storage. Previous federal regulations provided that the platform cannot be capable of stowing a weight greater than 50 pounds. Wheelchair lifts typically complied with this regulation by detecting an increase in dynamic pressure during a stow attempt of the wheelchair lift. During a stow attempt, the added weight on the platform is transferred through the linkage of the platform to the parallelogram mechanism causing an increase in the dynamic pressure of the hydraulic circuit, i.e., the pressure of the hydraulic circuit during movement of the lift. The increase in dynamic pressure triggers a pressure switch to prevent the stowage operation of the lift. The pressure switch is set to actuate or trigger at a pressure level that is higher than the dynamic pressure of the hydraulic circuit when no weight is positioned on the platform. Wheelchair lifts could comply with the previous regulation if the lift would be prevented from stowing with a 50 pound weight placed at the centroid of the platform. However, a new regulation was promulgated which provides that the platform cannot be capable of stowing a weight greater than 50 pounds positioned anywhere on the platform. Thus, the wheelchair lift must be capable of detecting a weight positioned at various points on the platform rather than at the centroid.
As the 50 pound weight is moved closer to the platform pivot, detecting the weight on the platform becomes more difficult by measuring an increase in the dynamic pressure during a stow attempt. If the 50 pound weight is placed at the platform pivot or behind the platform pivot, detecting the 50 pounds by measuring an increase in dynamic pressure is substantially ineffective. For instance, if the 50 pound weight is located 24 inches from the pivot point, the resulting moment required to fold the platform is approximately 1200 inch-lbs. However, if the 50 pound weight is moved to a point that is 3 inches from the pivot point, the resulting moment required to fold the platform is approximately 150 inch-lbs. The hydraulic circuit pressure required to fold the platform in the latter situation may be 800 psi, whereas the pressure required to fold the platform in the former situation may be 1100 psi. Detecting weight on the platform becomes more difficult as the pressure required to fold the platform is closer a hydraulic circuit pressure level that exists when there is no weight positioned on the platform during a stow attempt. In other words, if the weight is positioned closer to the pivot point, the difference in the dynamic pressure of the hydraulic circuit with weight and without weight on the platform becomes smaller, which can result in false readings that weight is present on the platform.